2d-3d GPr Across The mT. Pollino And cAsTroVillAri fAulTs (souThern cAlAbriA): driVinG PAleo-seismoloGy reseArches in A comPlex siTe To infer quATernAry eArThquAkesm. ercoli1, c. Pauselli1, e. forte2

1Dipartimento di Scienze della Terra, Università di Perugia, Italy2Dept. of Mathematics and Geosciences, Università di Trieste, Italy

Introduction. The definition of the seismic hazard of major seismogenic fault zones is often reached with the support of geophysical investigations to the geological and paleoseismological studies. Over the last years, paleoseismology investigates the subsurface through trenches realized across the fault branches (Galli et al., 2008) and employing geophysical techniques like 2D-3D Ground Penetrating Radar (GPR) (Liner and Liner, 1997; Mc Clymont et al., 2008; Pauselli et al., 2010, Ercoli et al., 2013). A multidisciplinary approach is particularly recommended on areas classified seismically “silent” from relatively long time and representing “seismic gaps”. On the other hand, these structures have morphological, geological and structural evidences compatible with the occurrence of past strong earthquakes, therefore any faults inside these “gaps” can be particular interesting for geoscientists in order to assess the seismic hazard. The knowledge of the characteristics and of the seismic behavior of the single fault segments, is fundamental to infer their evolution in the Quaternary age, to delineate which portion shows the highest probability of generate strong events, and to plan mitigation efforts for the area. The Mt. Pollino region can be classified as one of the most apparent “seismic gaps” along the central and southern Apennines seismogenic fault zone (Cinti et al., 1997; Michetti et al., 1997; Valensise and Pantosti, 2001), due to the historical seismicity and the poor “recent” instrumental activity consisting in small/moderate magnitude earthquakes (Ferreli et al., 1994). Anyway, a recent paroxystic seismic activity, characterized by about 3400 seismic events of M>1, including an earthquake of M = 5.0 (2012/10/26, Mormanno) have been recorded during the last two years on the area surrounding Castrovillari, (ISIDe Working Group, INGV, 2010). The Project S1 “Miglioramento delle conoscenze per la definizione del potenziale sismogenetico” (Agreement INGV-DPC 2012-2013, https://sites.google.com/site/progettisismologici/progetto-s1) is a recent integrated project started with two main aims: 1) to improve the base-knowledge for assessing the seismogenic potential of some areas considered of significant interest by the Project Board. 2) to develop innovative methodologies for the study of active faults, with a quantitative approach. The Pollino area, due to these uncertainties in the definition of the seismic hazard, is one of the zones studied during the last year. Among a broad spectrum of research topics and units, the project includes the GPR fault imaging on several sites close to Castrovillari, having different purposes: 1) to define the location and the geologic characteristics of the Quaternary faults in well-know sites; 2) to detect evidences of a geophysical signature interpretable with recent faulting extending the GPR surveys to new study sites; 3) to map these geological structures on thematic maps; 4) to support and drive new paleoseismological surveys.

We collected 2D-3D GPR data on the Mt. Pollino and the Castrovillari Faults (Fig. 1) in several sites. A first 2D-GPR survey was done at the “Grotta Carbone” site in order to calibrate the data with available trench logs (Michetti et al., 1997). The surveys have been extended to

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other places, like the eastward fault termination close to Civita, finally making an extensive 2D-3D GPR survey to a Southern branch of the Castrovillari fault. In particular the acquisition of 3D data was done to analyze a wider area with a dense recording grids (Grasmueck et al., 2005), in order to provide a data volume suitable to study the geological structure in their actual geometry. We obtained suitable a-priori information then used by the paleoseismologists to efficiently plan and drive the trench excavation.

Geological outline. Geologists and seismologists consider both the Mt. Pollino and the Castrovillari faults as two active, but “silent” structures, anyway capable to potentially generate strong earthquakes. Thanks to geomorphological field observations, paleoseismological analysis and comparison with surface faulting events in the Apennines and nearby regions, the estimated Mmax could be about 6.5-7 degrees. The Pollino area is located in the Calabria region (Southern Italy) and divides the NW-SE trending Southern Apennines from the Calabrian Arc. A crustal extension during the Quaternary age affected the area, generating tectonic basins like the Northest Mercure Basin and the southern Morano – Castrovillari Basin. The area represents the most significant seismicity gap within the southern Apennines (Michetti et al., 1997; Cinti et al., 2002), like analogous “silent” structures in the Central Apennines (Galli et al., 2008) because seismological data don’t show historical record of seismic events of M > 5. The W-NW “Pollino fault” and the N-S trending Frascineto/Castrovillari faults (Fig. 1), bound the hangingwall hosts named Morano – Castrovillari basin, filled by about 900 m of sedimentary units like marine rocks (upper Pliocene-lower Pleistocene) and continental deposits (middle Pleistocene to Holocene). These two faults are among the major Quaternary normal faults of the area showing paleoseismological evidences of late Quaternary activity (Michetti et al., 1997; Cinti et al., 1997, 2002). Recently, a seismic sequence including an earthquake of M = 5.0 (2012/10/26) struck in particular the western part of the region (ISIDe Working Group, INGV, 2010).

Fig. 1 – View of the study area. The Castrovillari basin shows the main faults alignments with the red lines, highlighted by morphologic fault scarps. The orange points indicate the earthquake locations and magnitude (M > 2.5) occurred between 2011-2013, whilst the two orange stars represent two events with M > 4 (Mormanno, M= 5.0, Morano Calabro, M = 4.3), extracted by the Iside Database. The circles highlight the three principal surveysites on which GPR data were recorded during the project.

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2D GPR Survey across the Mt. Pollino Fault (Grotta Carbone site). The “Grotta Carbone” site was chosen for a first 2D-GPR survey across a sector of the Mt. Pollino Fault, due to the availability of past paleoseismological analysis (Fig. 1, Ferreli et al., 1994; Michetti et al., 1997), used to validate the GPR data. More than 10 high-resolution 2D GPR profiles were recorded using a Zond 12e radar system; the profiles have been recorded parallel and very close to the two original trench tracks. 300 and 500 MHz shielded antennas guaranteed an optimal imaging of the fault zone, showing the best trade-off between resolution and investigation depth. The GPR lines were recorded intercepting transversally the fault and to check the lateral extension and other possible branches. The used acquisition parameters are summarized on Tab. 1. A Topcon GR-5 GNSS receiver, connected directly to the GPR system has been employed for the positioning with a centimetric accuracy. The data processing was done using a commercial software (Tab. 1). A recovery function was applied to compensate for the attenuation losses, preserving both lateral and vertical amplitude contrasts. A frequency bandpass filter and 2D average filter were used to reduce noise components and a background removal just on the 500 MHz profiles to remove horizontal ringing. After an accurate topographic correction, a time-depth conversion was done using a constant velocity of 0.08 m/ns, estimated by diffraction hyperbola analysis and tying the radar units with the stratigraphic logs. Finally, we calculated the “Envelope” attribute (Chopra and Marfurt, 2005; Forte et al., 2012), then plotted over the same processed radargram, to provides additional information for data analysis and interpretation, enhancing the high-resolution imaging of the fault zone and sedimentary structures.

Fig. 2 – a) Envelope attribute calculated for the Grotta Carbone profile and plotted under the processed profile after the time-depth conversion. The discontinuities are enhanced by the combination of different amplitudes and layers bedding, simplifying and improving the overall accuracy in the radargram interpretation. b) log of the trench T2 (from Michetti et al., 1997) is here re-proposed for direct comparison with the radar data.

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2D-3D GPR survey across the Castrovillari Fault. A wide number of 2D GPR profiles and a 3D GPR volume were acquired during the last field survey across the Castrovillari fault (Fig. 1). The survey site has been identified by the UR of Dr. Francesca Cinti (INGV), after an accurate geological survey on the area. The same was employed for the data recording. In the present paper only some main results on the preliminary 2D 300 MHz profiles are reported. The 2D profile in Fig. 3a was recorded close to an outcrop showing the studied fault and close to the area on which the 3D GPR acquisition grid was materialized. An inter-trace distance of 0.01 m (Δx) has been used for data recording with both the used antennas. The data have been first processed with a basic flow. The velocity estimation has been defined through an hyperbola diffraction analysis (“hyperbola fitting”): a resulting average velocity value of 0,095 m/ns has been estimated for the investigated subsoil, and then used for the final time to depth conversion. A static (“topographic”) correction and a f-k migration algorithm has been applied to the data employing the same velocity, in order to restore the true dips to the reflectors and collapse the hyperbolic diffractions. The fault zone has been better focused in a narrow line at about 22 m, providing reliable information on its geometric characteristics. Fig. 3a illustrates the processed un-migrated profile, more suitable to highlight the fault zone even using the diffractions. Then, a 3D GPR volume has been acquired employing a 300 MHz antenna, employing a distance of 0.1 m between profiles and an inter-trace distance of 0.01 m. The size of the acquisition grid was 5,5 x 20 m, respectively in NW-SE and NE-SW directions, consisting in 56 2D GPR profiles each of 2000 (Fig. 3). The flow used for the data processing was analogous to the one already described for the 2D lines.

Tab. 1 – Table summarizing the main acquisition parameters used for the 2D/3D GPR survey done on the Castrovillary area.

Acquisition parameter Time

window(ns)

Estimated RelativeDielectric

Permittivity ( r)

Total number of samples for trace

(n°)

Inter-trace distance

(m)

Profile distance(3D data)

(m)Antenna Central Frequency

300 MHz 200/300 14 (Pollino) 512/1024 0.05/0.01 0.10 (Castrovillari)

500 MHz 100/200 10 (Castrovillari) 512/1024 0.01 /

Integrated interpretation of the 2D/3D data. The available trench data in Fig. 2b across the Mt. Pollino fault (Grotta Carbone site) have been extended in length as well as in depth by the GPR profiles, imaging the geological structures and the fault zone with high-resolution. The original stratigraphic information about the units described by Michetti et al. (1997) as alluvial fan deposits (Pleistocene) were highlighted in depth by a discontinuity dividing gentle inclined bedding on underlying dipper layers (Fig. 2a, 100 ns): these can be interpreted as an older unit belonging to the alluvial fan deposits, like a seismic “bedrock”, compatible with the high probing depth investigated. The attribute analysis helps the visual interpretation of the tectonic discontinuities and the sedimentary units, which can be accurately deduced and easily followed. The fault offset can be estimated from some steps separating the colluvial materials on the fault hanging wall, that looks like “transparent” to the radar energy due to the strong reflection of the continuous basal reflector, comparable with the one of the foot wall units (Fig. 2a,b). Some geophysical signatures already observed in literature by some authors (Liner and Liner, 1997; Bano et al., 2002; Pauselli et al., 2010; Ercoli et al., 2013) were identified, like relative differences in signal amplitude, attenuation of the radar units, interruptions of the lateral continuity and dip of the reflectors, diffraction hyperbolas (in un-migrated data) and

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also a different character of the direct arrivals. The different geological units can be therefore interpreted and mapped, and some parameters can be extracted by the radargrams, like the layers thickness, bedding and the fault offset, of about 1 meter at 23 m along the profile (white arrow on the right in Fig. 2a).

The 300 MHz radargram here reported for the Castrovillari survey shows a good S/N ratio and signals suitable for an efficient geological interpretation (Fig. 3a). This can be ideally divided in four sectors, having different characteristics: in the SW sector between 0-20 m (black box) some W-SW dipping reflectors are visible over a sub-horizontal unit at about 2 m of depth from the topographic surface. That geometry is compatible with a transversal section of the lentiform sedimentary bodies belonging to N-S alluvial fan: such layers are also well-visible on outcrops close to the survey site. The red box highlights an area characterized by the presence of some diffractions (in unmigrated data) and an interruption of the lateral continuity among the reflectors, which represents a peculiar geophysical signature of a fault. The diffractions are probably generated by the high dipping “fault plane” and the lateral stratigraphic contact between two materials having different dielectric properties, probably including cemented filling and/or cemented “blocks” within the faulted units (like in the outcrop picture in Fig. 3b).The fault is highlighted also by the different electrical response of the subsurface within the foot-wall and hanging-wall sectors, and by the stronger signal attenuation compared with the surrounding areas. The unit enclosed in the green box (Fig. 3a) is high reflecting with sub-horizontal layers, gently NE-dipping: this is interpretable as a sort of “bedrock”, probably related to a cemented unit. Finally, some weak reflecting reflectors are imaged in the Eastern sector of the radargram (fault hanging-wall, orange box about between

Fig. 3 – 2D and 3D GPR data recorded on the Castrovillari site. a) Un-migrated 2D profile shows different areas within the dashed boxes, where the fault zone, characterized by a well defined signature, has been highlighted by the red one. b) Field picture reporting the fault on the outcrop. c) The 3D data show both the fault located at about 10 m along the GPR volume (X axis), and the different units detected in this complex site, like the shallow backfill layers and the colluvial/alluvial deposits corresponding with the attenuated side.

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28-38 m, Fig. 3a). These are layered over the high reflective unit and can be correlated with the ochre sands mapped on the nearby outcrop (Fig. 3b). The combined analysis of the 3D GPR volume on vertical sections and depth slices provide an optimal visualization and imaging of the structures, highlighting a sector of the volume between the interval 2.5-5.5 m in the NW-SE direction (Y axes), where the geological structures are clearly defined (limited by a blue dashed line in fig. 3). In the area ranging from 0-3 m, with the exception of the shallower layers (0-40 ns), the structures are not well defined in depth due to a strong attenuation of the GPR signal. The boundary between the two zones in the 3D volume is sharp and easily detectable, and it separates two units showing very different electric properties. The North sector of the volume, located close to the scarp in field, highlights the discontinuity between the reflectors interpretable with the fault zone at about 10 m along the profiles (along the SW-NE direction, from SW to NE). In addition to this discontinuity, some diffractions already described in the 2D profile, represent features defining a characteristic geophysical signature of the fault zone. Fig. 3c reports a close-up obtained trough a depth-slice at about 2 m. Though the management and the interpretation of the whole volume at different depth, the fault zone visible also in the nearby outcrop was localized with high-resolution, extending in depth the morphological and geological surface data and providing new data useful for further paleoseismological analysis. Some quantitative and geometrical characteristics of the fault zone and others considerations about the sedimentary units can be done: a mean dip-direction has been estimated in about 235°, whilst the dip angle was close to 70°. These values are in agree with ones provided after the further ground-truth done on the site by the INGV research group of paleo-seismologists (UR Cinti). The new trench was focused across the two areas showing different GPR signature, intercepting both the fault and the sharp boundary highlighted by the 3D GPR volume, ensuring a successful data validation about the geometric characteristic of the fault. The blue dashed boundary therefore actually represents the separation between the natural subsoil nearby the scarp (fan deposits) and progressively thicker backfill layers located above the road on the survey site. The latter resulted stratified above a colluvial-fluvial unit originated by the erosion and re-sedimentation of a NE-SW stream, which generated the small NE-SW valley where the 3D data were acquired.

Discussion and conclusions. The 2D/3D GPR “Grotta Carbone” and “Castrovillari” datasets highlighted characteristic geophysical signature of fault zones, efficiently defining the tectonic structure through some features like lateral truncations of layers, different dip of the reflectors on the two sides of the fault, presence of diffraction hyperbolas (un-migrated data) and a narrow strong attenuated zone. Some others considerations can be done on the areas surrounding the fault zone. The survey site shows reflectors having a W-dipping direction trend, excepted for the NE sector of the radargrams, where the layers are gently E-dipping. The 3D GPR analysis highlights how the S-SE sector of the acquisition is characterized by electrical properties of the investigated materials, which prevent to image geological structures in the S-SE side due to strong signal attenuation. Close to the outcrop (scarp), already from about halfof the survey grid, both the fault zone and the sedimentary layers are well imaged by the data.The subsoil between the scarp and about 3 m towards South, is characterized in the first 3 m ofdepth by strong E-dipping reflectors. Some vertical sections and the depth-slices illustrate thisunit is laterally extending in depth but then abruptly interrupted, probably due to the erosion/ re-sedimentation operated by a pre-existent stream. In the shallow part a boundary withsome backfill layers filling the valley, probably produce high attenuation and low amplitudereflections, reducing the investigation depth and complicating the visualization/interpretationwithin this sector. The structures interpreted in the 2D GPR sections and 3D GPR volume, likethe fault zone, some important layers and stratigraphic units have been successively validatedby the trench excavation. The trench revealed a correct geophysical interpretation not only ofthe geometrical characteristic of the fault (strike, dip direction, dip angle), but also of the lateralboundary visible in the depth-slices (backfill, alluvial deposits…etc). The survey represents an

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effective case history of GPR application on the study of geological discontinuity as faults and fractures, but also efficient in the characterization of the related sedimentary structures: the technique is fundamental to provide both suitable “a-priori” information to plan and optimize a further paleoseismological survey and to potentially extend the data on a wider area. This work therefore represents a successful example of how a GPR survey can be for paleoseismological studies, providing a detailed geological model of the subsurface by the interpretation of high-resolution 3D GPR volume and 2D sections, also improved by a basic attributes calculation like in the “Grotta Carbone” case. It represents a valuable tool to detect, though the interpretation of typical radar signature, a Quaternary fault zone and the sedimentary structures, both to define in a non invasive way, the optimal area for future trenches, and to extrapolate complementary information to derive an improved geological model. These detailed results provided on tectonic and stratigraphic information are comparable and complementary with the ones provided by the trenches stratigraphies, therefore suitable to the paleoseismologists to efficiently plan and drive a new trench excavation on the studied Castrovillari site. GPR data can be extensively used where there is a lack of geological information and the results of data interpretation can improve the definition of the seismic hazard of an area, proving a base suitable to plan valid actions for the prevention and the mitigation of the earthquake risks. The interpreted faults can be then potentially added to detailed structural and geological maps thanks to the centimetric accuracy of the geographic coordinates provided by the GNSS receiver linked to a NRTK correction.Acknowledgements. We wish to thank Leonardo Speziali, for his indispensable contribution during the field surveys, Alessandro Maria Michetti and Livio Franz for the preliminary field trip on the study site and for the geological framework provided, the whole Cinti UR for sharing the trench excavation data and Roberto Volpe for the help provided in data acquisition and processing.

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